Electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, batteries, and the like, often employ ion conductive membranes (ICM) as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with cathode and anode electrodes, and transports ions such as protons that are formed at the anode to the cathode, allowing a current of electrons to flow in an external circuit connecting the electrodes. The ICM may contain a polymeric electrolyte. Proton- or cation-conductive ICMs may contain polymeric electrolytes bearing anionic or acidic functional groups such as --CO.sub.2 H, --SO.sub.3 H, or other groups bearing acidic protons.
It is a common practice in the industry to use uncrosslinked sulfonated polymers in ICMs, such as Nafion.TM. (DuPont Chemicals, Inc., Wilmington, Del.) which is made of copolymers of tetrafluoroethylene and perfluorovinylether sulfonic acid. However, commercially available ICMs are not entirely satisfactory in meeting the performance demands of fuel cells. For example, Nafion.TM. membranes have inherent structural weakness at low equivalent weights. Nafion.TM. membranes are not generally available at thicknesses of less than 50 .mu.m. Thinner Nafion.TM. membranes would require reinforcement, thus defeating the purpose of a thin membrane by increasing the overall thickness as well as increasing the electrical resistance of the membrane. While Nafion.TM. membranes with lower equivalent weight can achieve lower electrical resistance, these lower equivalent weight membranes are also structurally weaker, and therefore still do not obviate the need for reinforcement.
Buchi et al., J. Electrochem. Soc., 142(9) at 3044 (Sep. 1995), discloses proton exchange membranes made by sulfonating a crosslinked polyolefin-polystyrene copolymer. The polymer is crosslinked during polymerization by addition of divinyl benzene.
U.S. Pat. No. 5,438,082 discloses a method of crosslinking a sulfonated aromatic polyether ketone using a two-step, two-part crosslinker. The crosslinking molecule is difunctional, comprising an amine function and a crosslinkable constituent. The crosslinker is attached by its amine function to a sulfonylchloride on the polymer to form a sulfonylamide, which may be a hydrolytically unstable group. This operation consumes an ion conducting sulfonic acid function. After the modified polymer is cast into a membrane, the crosslinkable constituents are joined to form crosslinks. This reference does not disclose or suggest a crosslinker that maintains the acidity of the membrane by forming stable yet highly acidic linkages, such as imide linkages.
U.S. Pat. No. 5,468,574 and WO 97/19,480 (published May 29, 1997) disclose that certain sulfonated polymers will form direct bonds between sulfonate groups upon heating. These references do not disclose the use of any crosslinker. WO 97/19,480 emphasizes that this method requires the sacrifice of sulfonic acid groups and a resulting loss of acidity in the membrane.